The present application claims priority to Japanese Application(s) No(s). P2001-029533 filed Feb. 6, 2001, and P2002-006851 filed Jan. 16, 2002, which application(s) is/are incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
The present invention relates to a spontaneous luminescence type organic electroluminescent device used as a display unit for a display unit, for example, a color display, and to a display unit using the organic electroluminescent device.
2. Description of the Related Art
In recent years, regarding multimedia-oriented products and other products, the interface between human and machine has become more important. In order to efficiently operate the machine, adequate amount of information must be concisely and instantaneously retrieved from the machine with no error. Therefore, research has been made relative to displays and other various display units.
Accompanying miniaturization of machines, requirements for miniaturized and low-profile display units have become more intensified day by day. Under these circumstances, regarding most laptop information processing devices and other miniaturized products, for example, miniature televisions, watches, and desk calculators, liquid crystal displays have been used as the interfaces thereof.
The liquid crystal display has been researched almost exclusively as an essential interface for display units from miniaturized to large-capacity taking advantage of the characteristics of the liquid crystal, that is, low-voltage drive and low power consumption.
However, since the liquid crystal display is of photoreceptive type, backlight is indispensable, and an electric power required for driving the backlight is larger than that for the liquid crystal. Consequently, there is a limitation in power supply even though a storage battery is built in and, therefore, a problem of restriction in the use, for example, reduction of uptime, may occur.
Furthermore, since liquid crystal display is the display based on the status of orientation of the liquid crystal molecule, contrast varies depending on angles even within the viewing angle thereof. Consequently, the viewing angle is reduced and, therefore, the liquid crystal display is not suitable for a large display, etc. In addition, since the orientation speed of the liquid crystal display is low, it is not suitable for displaying moving images.
On the other hand, from the viewpoint of drive system, although an active matrix system of the liquid crystal display exhibits an adequate response speed for handling moving images, since a TFT driving circuit is used, it is difficult to increase a screen size because of pixel defect, and it is inadvisable from the viewpoint of cost reduction as well. Regarding a passive matrix system that is another drive system, the cost is low, and it is relatively easy to increase the screen size in a manner opposite to that of the active matrix system. However, it cannot exhibit an adequate response speed for handling moving images.
Aside from such a liquid crystal display unit, display units using spontaneous luminescence type elements, for example, plasma display elements, inorganic electroluminescent devices, and organic electroluminescent devices, have been researched.
Among these, the plasma display element uses plasma emission in gas for display, and is suitable for an increase in size and increase in capacity. However, since there are problems in reduction of thickness and cost, and high-voltage bias is required for driving, it is not suitable for portable devices.
Regarding the inorganic electroluminescent device, although a green-emitting display, etc., were commercialized, since a few hundred voltages were required for alternating current bias drive similarly to the plasma display element, they were not accepted by users. Although emission of the primary colors RGB required for a color display has achieved success as a result of advance in technique after that, since inorganic materials are indispensable in the configuration, control of emission wavelength, etc., by molecular design, etc., is impossible and, therefore, it is expected that full colorization will be attended with difficulties.
On the other hand, the organic electroluminescent device uses electroluminescence due to an organic compound. This phenomenon was already discovered about thirty-odd years ago from now. That is, in the first half of the 1960s, when a carrier was injected into an anthracene single crystal which exhibited an intense fluorescence, occurrence of specific emission phenomenon (due to induction of luminescence) was observed. Since then, the organic electroluminescent device was the focus of study for a long time. However, at any rate, since it had low brightness and monochrome, and a single crystal was used, technological emphasis was primarily laid on the carrier injection into the organic material and, therefore, it still remained in the realm of the stage of basic study.
However, in 1987, Tang et al. of Eastman Kodak Company disclosed an organic electroluminescent device having a laminated structure including an amorphous luminescent layer capable of low-voltage drive and high-brightness luminescence. Since then, considerable research has been made until now in various areas regarding luminescence, stability, increase in brightness, laminated structure, and manufacturing method related to the primary colors RGB.
Herein, in order to realize the full-color display, stable-RGB luminescent elements with high color purity are indispensable. For this reason, in the area of the organic electroluminescent device as well, considerable research and development have been made regarding luminescent materials and elements having a chromaticity based on NTSC (National Television System Committee) standard or sRGB (Standard RGB).
However, no blue-emitting material and element satisfying the blue chromaticity (0.14, 0.07) based on the NTSC standard or blue chromaticity (0.15, 0.06) based on the sRGB have been achieved.
For example, although Hosokawa et al. disclosed a blue-emitting organic electroluminescent device (hereafter referred to as blue-emitting element) using a DPVBi-based material in the literature “Asia Display'95, 269, (1995)”, as indicated by the point a in the CIE chromaticity diagram shown in FIG. 2, the chromaticity of this blue-emitting element is on the order of a(0.16, 0.19). The chromaticity range based on the sRGB is also shown in the chromaticity diagram.
In October, 2000, SANYO Electric Co., Ltd. disclosed an active matrix TFT full-color display of 5.5 inches. The chromaticity of the blue pixel thereof is b(0.17, 0.17) as indicated by the point b in FIG. 2.
It is known that the stability of the film structure of an organic thin film constituting the element is important for the stability and durability of an organic electroluminescent device. Usually, since the organic thin film is formed in an amorphous state, it can be said that a material capable of stably maintaining the amorphous state, that is, the material having a high glass transition temperature (Tg), is superior heat-resistant material. As the method for improving the Tg of the material constituting the organic thin film, a method, in which a cohesive force between molecules is reduced, and crystallinity is reduced by introducing a branch or nonplanar property in the molecular structure of compound, is adopted. Typical examples thereof include a compound having a starburst structure or spiro structure.
In particular, the spiro structure is a remarkably nonplanar molecular structure and, therefore, a material having high heat resistance can be developed. From such a viewpoint, Hoechst AG suggested a compound including the spiro structure which reduces cohesive force between molecules and reduces crystallinity by introducing a branch or nonplanar property in the molecular structure of compound (refer to a literature “Polymer Preprints38 (1997) 349”). In Japanese Unexamined Patent Application Publication No. 7-278537, it is described that an organic electroluminescent device using 2,2′,7,7′-tetrakis(biphenyl)-9,9′-spirobifluorene which is a compound having such a Spiro structure exhibits blue emission. In a literature “Synthetic Metals91, 209 (1997)”, it is reported that the chromaticity of an element using this compound as a material is c(0.18, 0.15) indicated by the point c shown in FIG. 2.
In addition to these, there is a description regarding an organic electroluminescent device using 2,2′,7,7′-tetrakis(biphenyl)-9,9′-spirobifluorene having the spiro structure and Alq3 in International Patent Application PCT/JP95/01539. This organic electroluminescent device is a green-emitting element using this spiro compound as a hole transportation material and using Alq3 as a luminescent layer.
As described above, although considerable research and development have been made regarding stable luminescent materials and elements emitting each RGB color with a high color purity in order to realize a full-color display, especially, the chromaticity of blue-emitting material and element is inadequate, and no luminescent element described above has satisfied the blue chromaticity (0.14, 0.07) based on the NTSC standard or blue chromaticity (0.15, 0.06) based on the sRGB.
Consequently, regarding the display unit using the organic electroluminescent device, it has been difficult to realize full-color display with high color expressivity.